Amorphous carbon coatings for fuel cell bipolar plates

ABSTRACT

A flow field plate for fuel cell applications includes a metal with a non-crystalline carbon layer disposed over at least a portion of the metal plate. The non-crystalline carbon layer includes an activated surface which is hydrophilic. Moreover, the flow field plate is included in a fuel cell with a minimal increase in contact resistance. Methods for forming the flow field plates are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 12/181,864filed Jul. 29, 2008, now U.S. Pat. No. 8,497,050 issued Jul. 30, 2013,the disclosure of which is hereby incorporated in its entirety byreference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

In at least one embodiment, the present invention is related to bipolarplates used in PEM fuel cells

2. Background Art

Fuel cells are used as an electrical power source in many applications.In particular, fuel cells are proposed for use in automobiles to replaceinternal combustion engines. A commonly used fuel cell design uses asolid polymer electrolyte (“SPE”) membrane or proton exchange membrane(“PEM”), to provide ion transport between the anode and cathode.

In proton exchange membrane type fuel cells, hydrogen is supplied to theanode as fuel and oxygen is supplied to the cathode as the oxidant. Theoxygen can either be in pure form (O₂) or air (a mixture of O₂ and N₂).PEM fuel cells typically have a membrane electrode assembly (“MEA”) inwhich a solid polymer membrane has an anode catalyst on one face, and acathode catalyst on the opposite face. The anode and cathode layers of atypical PEM fuel cell are formed of porous conductive materials, such aswoven graphite, graphitized sheets, or carbon paper to enable the fuelto disperse over the surface of the membrane facing the fuel supplyelectrode. Each electrode has finely divided catalyst particles (forexample, platinum particles), supported on carbon particles, to promoteoxidation of hydrogen at the anode and reduction of oxygen at thecathode. Protons flow from the anode through the ionically conductivepolymer membrane to the cathode where they combine with oxygen to formwater, which is discharged from the cell. The MEA is sandwiched betweena pair of porous gas diffusion layers (“GDL”), which in turn aresandwiched between a pair of non-porous, electrically conductiveelements or plates. The plates function as current collectors for theanode and the cathode, and contain appropriate channels and openingsformed therein for distributing the fuel cell's gaseous reactants overthe surface of respective anode and cathode catalysts. In order toproduce electricity efficiently, the polymer electrolyte membrane of aPEM fuel cell must be thin, chemically stable, proton transmissive,non-electrically conductive and gas impermeable. In typicalapplications, fuel cells are provided in arrays of many individual fuelcell stacks in order to provide high levels of electrical power.

The electrically conductive plates currently used in fuel cells providea number of opportunities for improving fuel cell performance. Forexample, these metallic plates typically include a passive oxide film ontheir surfaces requiring electrically conductive coatings to minimizethe contact resistance. Such electrically conductive coatings includegold and polymeric carbon coatings. Typically, these coatings requireexpensive equipment that adds to the cost of the finished bipolar plate.Moreover, metallic bipolar plates are also subjected to corrosion duringoperation. Degradation mechanism includes the release of fluoride ionsfrom the polymeric electrolyte. Metal dissolution of the bipolar platestypically results in release of iron, chromium and nickel ions invarious oxidation states.

For water management, it is desirable for metal bipolar plates to have alow contact angle at the bipolar plate/water border; that is, a contactangle less than 40°. Titanium nitride coatings have been proposed ascorrosion-resistant plating for bipolar plates. Although titaniumnitride coatings are cost-effective, such coatings do not providesatisfactory protection for the bipolar plate material. Further,titanium nitride coatings develop relatively low water affinity with acontact angle close to 60°.

Accordingly, there is a need for improved methodology for lowering thecontact resistance at the surfaces of bipolar plates used in fuel cellapplications.

SUMMARY OF THE INVENTION

The present invention solves one or more problems of the prior art byproviding in at least one embodiment a flow field plate for use in fuelcell. The flow field plate of this embodiment comprises a metal platewith a non-crystalline carbon layer disposed over at least a portion ofthe metal plate.

In another embodiment, a fuel cell incorporating the flow field plateset forth above is provided. The fuel cell includes a first flow fieldplate with a surface coated with a non-crystalline carbon film. A firstcatalyst layer is disposed over the first flow field plate. A polymericelectrolyte is disposed over the first flow field plate, which a secondcatalyst layer over the polymeric electrolyte. Finally, a second flowfield plate is disposed over the second catalyst layer. Gas diffusionlayer are provided as needed.

In still another embodiment, a method for forming the flow field plateset forth above is provided. The method comprises depositing anon-crystalline carbon layer on a metallic plate followed by activatingthe carbon layer surface.

Other exemplary embodiments of the invention will become apparent fromthe detailed description provided hereinafter. It should be understoodthat the detailed description and specific examples, while disclosingexemplary embodiments of the invention, are intended for purposes ofillustration only and are not intended to limit the scope of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will become more fullyunderstood from the detailed description and the accompanying drawings,wherein:

FIG. 1A provides a cross sectional view of a fuel cell incorporating anexemplary embodiment of an non-crystalline carbon layer on a unipolarplate;

FIG. 1B provides a cross sectional view of a fuel cell incorporating anexemplary embodiment of an non-crystalline carbon layer on a bipolarplate;

FIG. 2 provides a cross sectional view of a bipolar plate channel coatedwith a non-crystalline carbon layer;

FIG. 3 provides a cross sectional view of a fuel cell incorporatinganother exemplary embodiment of an amorphous carbon layer on a bipolarplate;

FIGS. 4A and 4B provide a flowchart illustrating an exemplary method formaking a bipolar plate coated with a non-crystalline carbon layer;

FIG. 5 is a schematic illustration of a sputtering system used todeposit carbon layers;

FIG. 6A is a scanning electron micrograph of an un-plasma treated carbonlayer;

FIG. 6B is a scanning electron micrograph of plasma treated carbonlayer;

FIG. 7 provides a plot of contact resistance versus pressure for a fuelcell incorporating a carbon coated bipolar plate;

FIG. 8 is a plot of the high frequency resistance (HFR) verses currentdensity for fuel cells incorporating a carbon coated bipolar plate; and

FIG. 9 provides plots for three fuel cell in a stack of fuels cells toassess the wet-to-dry power stability.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Reference will now be made in detail to presently preferredcompositions, embodiments and methods of the present invention, whichconstitute the best modes of practicing the invention presently known tothe inventors. The Figures are not necessarily to scale. However, it isto be understood that the disclosed embodiments are merely exemplary ofthe invention that may be embodied in various and alternative forms.Therefore, specific details disclosed herein are not to be interpretedas limiting, but merely as a representative basis for any aspect of theinvention and/or as a representative basis for teaching one skilled inthe art to variously employ the present invention.

Except in the examples, or where otherwise expressly indicated, allnumerical quantities in this description indicating amounts of materialor conditions of reaction and/or use are to be understood as modified bythe word “about” in describing the broadest scope of the invention.Practice within the numerical limits stated is generally preferred.Also, unless expressly stated to the contrary: percent, “parts of,” andratio values are by weight; the term “polymer” includes “oligomer,”“copolymer,” “terpolymer,” and the like; the description of a group orclass of materials as suitable or preferred for a given purpose inconnection with the invention implies that mixtures of any two or moreof the members of the group or class are equally suitable or preferred;description of constituents in chemical terms refers to the constituentsat the time of addition to any combination specified in the description,and does not necessarily preclude chemical interactions among theconstituents of a mixture once mixed; the first definition of an acronymor other abbreviation applies to all subsequent uses herein of the sameabbreviation and applies mutatis mutandis to normal grammaticalvariations of the initially defined abbreviation; and, unless expresslystated to the contrary, measurement of a property is determined by thesame technique as previously or later referenced for the same property.

It is also to be understood that this invention is not limited to thespecific embodiments and methods described below, as specific componentsand/or conditions may, of course, vary. Furthermore, the terminologyused herein is used only for the purpose of describing particularembodiments of the present invention and is not intended to be limitingin any way.

It must also be noted that, as used in the specification and theappended claims, the singular form “a,” “an,” and “the” comprise pluralreferents unless the context clearly indicates otherwise. For example,reference to a component in the singular is intended to comprise aplurality of components.

Throughout this application where publications are referenced, thedisclosures of these publications in their entireties are herebyincorporated by reference into this application to more fully describethe state of the art to which this invention pertains.

The terms “roughness average” or “surface roughness average” as usedherein means arithmetic average of the absolute values of the profileheight deviations. The roughness average may be determined in accordancewith ANSI B46.1. The entire disclosure of this reference is herebyincorporated by reference.

The term “non-crystalline carbon layer” as used herein means a layercomprising at least 80 weight percent carbon with less than 10 weightpercent of the layer being crystalline. Typically, non-crystallinecarbon layers are at least 90 weight percent carbon with less than 5weight percent of the layer being crystalline. In a refinement,non-crystalline carbon layers are substantially amorphous carbon.

In an embodiment of the present invention, a flow filed plate for use infuel cell applications is provided. The flow field plate of thisembodiment comprises a metal plate with a non-crystalline carbon layerdisposed over at least a portion of the metal plate. The presentembodiment encompasses both unipolar and bipolar plates.

With reference to FIGS. 1A and 1B, a schematics cross section of fuelcells incorporating the flow field plates of this embodiment isprovided. Fuel cell 10 includes flow field plates 12, 14. Typically,flow field plates 12, 14 are made from a metal such as stainless steel.Flow field plate 12 includes surface 16 and surface 18. Surface 16defines channels 20 and lands 22. FIG. 1A provides a depiction in whichflow field plate 12 is a unipolar plate. FIG. 1B provides a depiction inwhich flow field plate 12 is a bipolar plate. In this variation, 18defines channels 24 and lands 26. Similarly, flow field 14 includessurface 30 and surface 32. Surface 30 defines channels 36 and lands 38.FIG. 1A provides a depiction in which flow field plate 14 is a unipolarplate. FIG. 1B provides a depiction in which surface 32 defines channels40 and lands 42.

Still referring to FIG. 1, non-crystalline carbon layer 50 is disposedover and contacts surface 16. In a variation, non-crystalline carbonlayer 50 includes surface 52 having a contact angle less than about 30degrees.

Still referring to FIG. 1, fuel cell 10 further includes gas diffusionlayer 60 and catalyst layers 62, 64. Polymeric ion conductive membrane70 is interposed between catalyst layers 62, 64. Finally, fuel cell 10also includes gas diffusion layer 72 positioned between catalyst layer64 and flow field plate 14.

In a variation of the present invention, a first gas is introduced intochannels 20 and a second gas is introduced into channels 36. Channels 20direct the flow of the first gas and channels 36 direct the flow of thesecond gas. In a typically fuel cell application, an oxygen-containinggas is introduced into channels 20 and a fuel is introduced intochannels 36. Examples of useful oxygen containing gases includemolecular oxygen (e.g., air). Examples of useful fuels include, but arenot limited to, hydrogen. When an oxygen-containing gas is introducedinto channels 20, water is usually produced as a by-product which mustbe removed via channels 20. In this variation, catalyst layer 62 is acathode catalyst layer and catalyst layer 64 is an anode catalyst layer.

With reference to FIG. 2, a magnified cross sectional view of channel 20is provided. Surfaces 80, 82, 84 of non-crystalline carbon layer 50provide exposed surfaces in channel 20. Advantageously, these surfacesof non-crystalline carbon layer 50 are hydrophilic, having a contactangle less than about 30 degrees. In another refinement, the contactangle is less than about 20 degrees. The hydrophilic nature ofnon-crystalline carbon layer 50 prevents water from agglomerating inchannels 20. In a refinement of the present embodiment, thehydrophilicity of non-crystalline carbon layer 50 is improved byactivating surface 52 (i.e., surfaces 80, 82, 84, 86). In a variation ofthe present embodiment, the surface is activated by a plasma (e.g., RFplasma, DC plasma, microwave plasma, hot filament plasma, open airplasma, and the like). In one refinement, the activation is accomplishedby exposing the non-crystalline carbon layers to a reactive oxygenplasma which would activate the non-crystalline carbon layers bybreaking bonds and forming hydroxyl, carboxyl and aldehyde functionalgroups. In another refinement, the post treatment is accomplished byexposing the non-crystalline carbon layers to reactive gases nitrogen,nitrous oxide, nitrogen dioxide, ammonia or mixture thereof, whichactivate the non-crystalline carbon layers by breaking bonds and formingnitrogen-based derivatives like amines, amide, and diazo functionalgroups. Accordingly, the post-treatment activation is able to increasethe amounts of nitrogen and/or oxygen in non-crystalline carbon layer50. In a further refinement, the amounts of nitrogen and oxygen inregions within several nanometers of surface 52. In another refinement,the activation of surface 52 results in an increase in porosity ascompared to the surface prior to activation. In a further refinement,surface 52 includes regions in which there at least 10 pores per μm² ofsurface area. Moreover, surface 52 includes on average at least 5 poresper μm² of surface area. The number of pores per μm² is calculated bycounting the number of pores in a given area observed in a scanningelectron micrograph.

The porosity of non-crystalline carbon layer 50 is also characterized bythe roughness average of surface 52. In a variation, the roughnessaverage of surface 52 is from about 200 to about 1000 nm. In anothervariation, the roughness average of surface 52 is from about 300 toabout 900 nm. In still another variation, the roughness average ofsurface 52 is from about 400 to about 700 nm.

The non-crystalline carbon layer of the present invention iselectrically conductive. The electrically conductivity ofnon-crystalline carbon layer 50 is such that the contact resistance offuel cell 10 is less than about 20 mohm-cm². In a variation of anexemplary embodiment, non-crystalline carbon layer 50 is doped in orderto increase the electrical conductivity. In one refinement,non-crystalline carbon layer 50 is doped. In a further refinement thedopant is a metal. Examples of suitable metal dopants include, but arenot limited to, Pt, Ir, Pd, Au, Ag, Co, Fe, Cu, Si, Ti, Zr, Al, Cr, Ni,Nb, Hb, Mo, W, and Ta. In another refinement, the dopant is a nonmetalsuch as nitrogen.

With reference to FIG. 3, a schematic cross section illustratingadditional surfaces of fuel cell bipolar plates coated withnon-crystalline carbon layers is provided. In this variation, one ormore of surfaces 18, 30, and 32 are coated with a non-crystalline carbonlayer 50. As set forth above, in connection with the description ofFIGS. 1A and 1B, fuel cell 10 includes flow field plates 12, 14. Bipolarplate 12 includes surface 16 and surface 18. Surface 16 defines channels20 and lands 22. Surface 18 defines channels 24 and lands 26. Similarly,bipolar plate 14 includes surface 30 and surface 32. Surface 30 defineschannels 36 and lands 38. Surface 32 defines channels 40 and lands 42.

Still referring to FIG. 3, non-crystalline carbon layer 50 is disposedover and contacts surface 16. In a variation, non-crystalline carbonlayer 50 includes surface 52 having a contact angle less than about 30degrees. Similarly, non-crystalline carbon layer 90 is disposed over andcontacts surface 18, non-crystalline carbon layer 92 is disposed overand contacts surface 30, and non-crystalline carbon layer 94 is disposedover and contacts surface 32. Fuel cell 10 further includes gasdiffusion layer 60 and catalyst layers 62, 64. Polymeric ion conductivemembrane 70 is interposed between catalyst layers 62, 64. Finally, fuelcell 10 also includes gas diffusion layer 72 positioned between catalystlayer 64 and bipolar plate 14. The details of non-crystalline carbonlayers 50, 92, 94 are set forth above in connections with thedescription of FIG. 1.

With reference to FIG. 4, a pictorial flowchart illustrating anexemplary method of forming the flow field plates set forth above isprovided. In step a), metal plate 12 is pre-conditioned prior todeposition of non-crystalline carbon layer 50. During suchpreconditioning oxides on the surface of metal plate 12 are typicallyremoved or at least reduced. Such pretreatment may include a cleaningstep. In step b), non-crystalline carbon layer 50 is deposited ontometal plate 12. The non-crystalline carbon layer may be formed by anumber of technologies known to those skilled in the art. Examples ofsuch technologies include, but are not limited to sputtering (e.g.,magnetron, unbalanced magnetron, etc), chemical vapor deposition (“CVD”)(e.g., low pressure CVD, atmospheric CVD, plasma enhanced CVD, laserassisted CVD, etc), evaporation (thermal, e-beam, arc evaporation, etc.)and the like. U.S. Pat. No. 5,314,716 discloses a CVD technique forforming non-crystalline carbon films. The entire disclosure of thepatent is hereby incorporated by reference. In step c), surface 52 ofnon-crystalline carbon layer 50 is activated. FIG. 4 depicts plasmaactivation via high density plasma 100. It should also be appreciatedthat additional methods of activation may be utilized. Such methodsinclude, but are not limited to, chemical activation such as treatment(e.g., etching) of the surface with an acid such as sulfuric acid,hydrofluoric acid, chromic acid, potassium permaganate, and the like.

In a variation of the present embodiment, non-crystalline carbon layersare deposited by sputtering. In one refinement, the carbon layers aredeposited using a closed field unbalanced magnetron system. For thispurpose, a variation of the method and apparatus set forth in U.S. Pat.No. 6,726,993 (the '993 patent). The entire disclosure of the '993patent is hereby incorporated by reference in its entirety.

With reference to FIG. 5, a refinement of a sputtering deposition systemfor depositing the non-crystalline carbon layers set forth above isprovided. FIG. 5 provides a schematic top view of the sputtering system.Sputtering system 102 includes deposition chamber 103 and sputteringtargets 104, 106, 108, 110 which are proximate to magnet sets 112, 114,116, 118. A magnetic field generated between the targets 104, 106, 108,110 is characterized with field lines extending to between themagnetrons forming a closed field. The closed field forms a barrier,which prevents the escape of electrons within plasma containing area122. Moreover, this configuration promotes ionization in the spacewithin the closed field with increased ion bombardment intensity. Highion current density is thereby achieved. Substrate 124 (i.e., metalplate 12) is held on platform 126 which rotates along direction d₁.Flipper 132 causes rotation of substrate 124 about direction d₂ during acycle of platform 126. In one example, sputtering targets 104, 106 arecarbon target while sputtering targets 108, 110 include metal dopants.Moreover, in this example, magnet sets 112, 114 provide a more intensemagnetic field than magnet sets 116, 118. This magnetic imbalance allowsfor less dopant to be sputtered than carbon. When system 102 isutilized, pre-conditioning step a) is advantageously performed by ionetching within deposition chamber 103. It has also been surprisinglydiscovered that placement of substrate 124 relatively close tosputtering targets 104, 106, 108, 110 tends to form non-crystallinecarbon as opposed to graphitic carbon. In one example, distance d₃,which is the closest distance from the center of substrate 124 to target104 during movement of the substrate is from about 5 cm to about 20 cm.

In one variation of the present, graphite targets are sputtered in achamber under the influence of a closed field unbalances magnetronfield. A useful sputtering system is the Teer UDP 650 system. Twographite targets are placed on strong magnetrons that may be sputteredat a current ranging from 5 A-50 A in a closed field magnetronarrangement. Two metal dopant targets are placed on two weaker magnets.The pressure in the sputter chamber may range from 1×10⁻⁶ to 1×10⁻⁴, abias voltage of −400V to −20V, pulse width of 250 nanosecond to 2,000nanosecond, and pulse DC at frequency rate of 400 KHz to 50 KHz, andargon flow rate of 200 sccm to 20 sccm for a time period of 10 minutesto 500 minutes. The film may be deposited in a thickness ranging from 5nm to 1,000 nm, or 10 nm to 50 nm.

FIGS. 6A and 6B provides scanning electron micrographs of amorphouscarbon layers deposited on stainless steel. FIG. 6A shows the anuntreated carbon surface while FIG. 6B provides the plasma treatedsurface. The samples are determined to be amorphous due to the absenceof peaks in an X-ray diffraction spectrograph. FIG. 7 provides a plot ofcontact resistance versus pressure. The pressure in these measurement isthe pressure applied to a multilayer structure including anon-crystalline carbon coated metal plate. At pressures representativeof fuel cell application (>100 psi), the contact resistance is observedto be less than 10 mohm-cm². Further evidence of the low contactresistances afforded by the carbon coated bipolar plates is demonstratedin FIG. 8. FIG. 8 is a plot of the high frequency resistance (HFR)verses current density for fuel cells incorporating the bipolar plate.Low HFRs are clearly demonstrated, particularly at current densitiesgreater than 0.4 A/cm².

The stability of fuel cell stacks that include fuel cells utilizing abipolar plate coated with a non-crystalline carbon layer is evaluated.FIG. 9 provide plots for three fuel cell in a stack of fuel cells toassess the wet-to-dry power stability. The fuel cells in this experimentare first preconditioned with a current density of 0.4 A/cm². At varioustimes the current densities are held constant. The over-lapping plots ateach of the test current densities is evidence that none of the cellsare failing. In particular, the over-lap of the plots at the 0.02 A/cm²current density is good evidence that the carbon layer is allowingeffective removal of water since at this low density theoxygen-containing feed gas is at a very low flow rate which couldpossibly make water removal difficult should the water not easily slidoff the channels in the bipolar plate.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

What is claimed:
 1. A method of making a flow field plate, the flowfield plate comprising: a metal plate having a first surface and asecond surface, the first surface defining a plurality of channels fordirecting flow a first gaseous composition; and a non-crystalline carbonlayer disposed over at least a portion of the metal plate, thenon-crystalline carbon layer having a carbon layer surface with acontact angle less than about 30 degrees, the method comprising:depositing the non-crystalline carbon layer on the metallic plate; andactivating the carbon layer surface; such that the carbon layer surfaceis an activated surface having nitrogen atoms.
 2. The method of claim 1wherein the carbon layer surface is activated with a plasma.
 3. Themethod of claim 2 wherein the plasma is DC plasma, an RF plasma,microwave plasma, or a hot filament plasma.
 4. The method of claim 1wherein the carbon layer surface is chemically activated.
 5. The methodof claim 1 wherein the carbon layer surface is pre-conditioned prior todeposition of the non-crystalline carbon layer.
 6. The method of claim 1wherein the contact angle less than 20 degrees.
 7. The method of claim 1wherein the carbon layer has wherein the carbon layer is doped with adopant atom.
 8. The method of claim 7 wherein the carbon layer haswherein the dopant atom is a metal atom.
 9. The method of claim 1wherein the carbon layer has wherein the non-crystalline carbon layercomprises amorphous carbon.
 10. The method of claim 1 wherein the carbonlayer has wherein the carbon layer includes less than about 10 weightpercent crystalline carbon.
 11. The method of claim 1 wherein the carbonlayer surface includes oxygen atoms.
 12. The method of claim 1 whereinthe carbon layer surface includes on average at least 5 pores per μm².13. The method of claim 1 wherein the carbon layer has a roughnessaverage from about 200 to about 1000 nm.
 14. The method of claim 1wherein the carbon layer has wherein the second surface defines a secondplurality of channels for directing flow of a second gaseouscomposition.
 15. The method of claim 1 wherein the carbon layer haswherein a second non-crystalline carbon layer is deposited on the secondsurface.